Your not near me are you?
I recently did a 10 plate avensis with identical fault,as you say the new unit is £900. he supplied his own 2nd hand unit £250 and we fitted it. the handbrake functioned once we had addapted the 2nd hand unit using g scan but the cars vin has to be inputted into the handbrake unit and its a one time deal so the handbrake wont release when clutch is released and the light is on the dash all the time. he has no option but to buy a new unit i am afraid.
sean